[netCDF] create a test for netCDF time series extraction and averaging for shapefiles

Hey Ines,

thanks a lot for coding this peace! When you find some time (it has time) then it would be great if you could include a small test. Description below:

The functions extract_timeseries_polygone and get_mean_polygone need to get tested in the CI workflow. A test should be implemented in the tests/test_aquipy.py